At 2:45 p.m. on July 2, an alarm sounded through the Wichita Falls wastewater treatment plant at 1005 River Lane.

The two plant operators on duty at the time knew what the alarm meant: sewage was leaking from the pipes in the basement onto the floor. Johnny Chavez and his co-worker descended toward the plant's basement. They peered through a grate and found that a valve connected to primary pump No. 2 had sprung a leak and coated the floor below it with at least a half-inch of goopy, black sludge.

It wasn't the first time operators had found a problem with the pump's valve. Repairs were ordered for it just a day before, and it had been worked on by mechanics six times in the last year.

Chavez paged David Sheppard, a senior mechanic with the city, to fix the valve.

'I told him that one of the primaries had busted. He had been here before and he said, 'I'll be there and soon as I can,'' Chavez said later that day in a police interview. 'I waited and I seen his truck pull up and I explained to him the situation. I guess he went and looked at it and came back and said to call his helper out, so we did.'

Safety protocols dictate that some repairs, such as the valve maintenance being ordered by the city, require two workers to be present — one to do the actual labor and one to keep an eye out for possible danger. Sheppard's 'helper,' city mechanic Daniel Arredondo, arrived at the plant shortly after being paged.

Sheppard and Arredondo navigated the metal staircase leading down to the pump, donning breathing masks and air tanks as a precaution.

They had been told it smelled 'gassy' down there.

Chavez told police he later saw Arredondo rinsing off Sheppard with a water hose — Sheppard had gotten splattered with sewage while making repairs. The two had remarked to Chavez that they were going to look a for fan to provide some ventilation and that they were going to try to find some more oxygen bottles for the breathing masks.

That was the last time Chavez, on anyone else, for that matter, would see Sheppard or Arredondo conscious. Chavez left the basement to finish his rounds at the plant, and after he did, the mechanics fell asleep.

They never woke up.

UNRESPONSIVE

At 5:01 p.m., an ambulance whirred to life and sped to 1005 River Lane, where it would be met by fire engines and Haz-mat crews. An operator at the city's wastewater treatment plant had called 911 dispatchers, reporting that two mechanics at the plant were unconscious and wouldn't wake up.

When first responders arrived at the plant and made entry into the basement, 'We found one person on the midpoint landing of the staircase and a second person sitting at the bottom of the staircase and in sludge,' a firefighter wrote in a statement that was later obtained by the Times Record News through a state open records request. 'Both persons were unconscious but gasping for air and snoring. They were blue-gray in color.'

Sheppard and Arredondo were loaded into ambulances and taken to United Regional hospital. Later they would be taken to Parkland hospital in Dallas for specialized care, but both died — Arredondo first, then Sheppard.

City officials determined the mechanics had inhaled a lethal volume of hydrogen sulfide, a deadly gas produced as a byproduct of decomposing human waste. The gas, which carries with it a distinct 'rotten egg' smell, can cause death or severe injury in humans with even limited exposure.

It was Chavez, the wastewater plant operator, who initially found Sheppard and Arredondo unresponsive in the basement. Chavez told a police officer conducting an investigation into the deaths that he thought the two were just taking a break.

'I went and checked on them to see if they were done, and that's when I found David on the first step,' he said. 'I said, 'Hey, David, you got to get up, man,' and he wouldn't respond. Then I looked down the grate and saw Daniel at the bottom and started hollering his name.'

Chavez had another plant operator call 911, and though first responders arrived quickly, Sheppard and Arredondo were already in bad shape, another firefighter noted in a report.

'I asked (a plant operator) if it was chlorine we were dealing with and he said no. It was a sludge that gave off some kind of gas,' the firefighter wrote. '(We) found the two men, who were not wearing self-contained breathing apparatuses, breathing but breaths were labored. An alarm was going off periodically that hampered communication.'

A cursory review of the scene showed the mechanics were in possession of four air tanks, though all of them contained lower-than-recommended air pressure levels. Further investigation revealed the men may not have checked air pressure levels in the tanks before checking them out from the city, which is protocol.

Sheppard and Arredondo did not take air quality measurements before beginning work, but neither did the plant operators, it appears.

It still is unknown why the mechanics removed their protective gear during the course of repairing the valve, though it could be because hydrogen sulfide is not a common threat in the plant's basement. Harold Burris, the city's wastewater collection/treatment superintendent, told an investigator it was 'unusual' for lethal levels of the gas to be present there.

More mechanics were called in to finish the valve repairs shortly after Sheppard and Arredono were hospitalized. It took about 10 minutes to fix the valve.

'The sludge was weird, though. There was hardly any on the ground but you could smell it outside the building,' Timothy Pecka, one of the replacement mechanics, told an investigator. 'It was worse than normal, and stained the concrete.'

Before the day was over, the city of Wichita Falls had already launched an investigation into the mechanics' injuries. Police officers took photos and interviewed witnesses, much like they would at a crime scene. An official from the city's Risk Management Department was also dispatched to conduct interviews and collect evidence in the case.

The most pressing question they needed to answer: Why did David Sheppard and Daniel Arredondo die?

BUSTED THEORIES

The next day, as part of the city's investigation, a wastewater plant administrator ventured down into the building's basement to record video footage of the scene. The camera lingered on areas of significance: the stairwell where Sheppard and Arredondo were found, enormous pipes stretching snakelike to industrial sewage pumps, concrete floors still slicked with sludge.

In one corner of the basement, lying on the floor in a pool of wastewater, were Sheppard's glasses, which apparently had fallen from the mechanic's face when he collapsed on the stairs above. The photographer illuminated them with a flashlight as machinery rumbled in the background.

There's little question about what killed Sheppard and Arredondo — officials concluded early on they had succumbed to hydrogen sulfide poisoning — but in the month since, investigators have been stumped as to how the gas became so concentrated in so little time. None of the theories that have been developed have panned out.

The city of Wichita Falls appears to have very little precedent to look to in determining the cause of the gas buildup. From 2011 to 2014, only 2 percent of local government workers in the U.S. have been killed on the job due to exposure to chemicals, and there are virtually no recent accounts of municipal employees dying from inhalation of hydrogen sulfide gas inside wastewater treatment facilities.

What happened to Sheppard and Arredondo is an anomaly among anomalies.

The Texas Hazard Communication Act dictates that cities report the chemical-related death of an employee to the Texas State Health Services Department. A subsequent investigation conducted by the department found that the city had trained Sheppard and Arredondo appropriately on the dangers of hazardous chemicals. It also found the city complied with state rules in posting information about chemicals at the plant.

But the agency's investigation stops there, a spokesman told the Times Record News. The state does not investigate the circumstances of a worker's death, nor does it conduct an inquiry into what caused the elevated levels of poison gas.

In fact, there are no independent, outside agencies who investigate work-related deaths of municipal employees in Texas. The Occupational Safety and Health Administration does not have jurisdiction in the incidents. When contacted by the city of Wichita Falls, the Texas Engineering Extension Service declined to open an investigation in the case.

The city, therefore, largely conducts its investigation alone. And for the workers' families, there is little other recourse except for inquiries brought by counsel through legal action.

John Sheppard, David Sheppard's cousin, told the Times Record News he's leery of a city investigation without independent oversight.

'Call me cynical, but Risk Management doing the investigation has appearances (that are) less than comforting,' he wrote in an email. 'David was not a haphazard or careless person when it came to serious matters like this event.'

As of Tuesday, no lawsuits had been filed in connection with the deaths.

For its part, the city has hired Frisco-based Freese and Nichols Engineering to determine the cause the of the gas buildup. But so far the company has not solved the mystery.

In the beginning stages of the probe, investigators developed three promising theories about what could have generated the hydrogen sulfide gas. Two of them involved the disposing of chemicals — glycerin and sulfur — into the wastewater treatment system. Another centered on construction being done on the plant.

But the city has no industrial companies pumping large volumes of sulfur into its system, and the glycerin is deposited at the plant in such a way that would not bring it into contact with the pipe Sheppard and Arredondo were repairing.

The groundwater at the plant's construction site showed elevated levels of sulfur, but that was also ruled out as a cause of the gas buildup.

With its best theories disproved, the city now aims to determine how common it is for hydrogen sulfide to be creeping through its wastewater system.

'We are conducting additional testing with the guidance of (an engineer) to determine if this an uncommon occurrence or if it is the norm for our wastewater,' said Russell Schreiber, public works director.

Per an internal city memorandum, plant workers now will be required to take air quality tests in the basement every two hours and also will have to carry gas detection devices into areas where hydrogen sulfide could be present. The memo states that permanent gas detection alarms may be installed in the facility.